Which is the Best Material FKM or PU for Hydraulic Pressure Oil?

Number of hits：142026-03-03 13:52:36　

FKM and PU seals behave very differently in hydraulic oil, and asking which one is “better” is misleading. Materials are not inherently good or bad; what matters is whether they match the temperature, pressure, and motion of the system.

FKM performs well in high‑temperature environments because it maintains stable volume, resists aging, and retains rebound properties even after long periods of compression. In hydraulic systems that operate around 120°C or higher, especially near pumps or power sources, PU often reaches its temperature limit. Once the temperature exceeds what the material can tolerate, elasticity drops, hardening accelerates, and sealing pressure decreases, leading to gradual leakage. FKM remains more stable under these conditions and offers a smoother, more predictable lifespan.

In high‑pressure reciprocating applications, the situation is reversed. PU has significantly higher mechanical strength, extrusion resistance, and wear resistance. Piston and rod seals in hydraulic cylinders experience pressure spikes and frequent impact loads. FKM, being a softer elastomer, is more prone to edge damage or tearing under these conditions. PU, with its higher hardness and tear resistance, performs better in high‑pressure dynamic environments.

Many seal failures are not caused by material defects but by incorrect selection. Using PU in a high‑temperature system often results in hardened lips within months, while replacing a high‑pressure seal with FKM can lead to edge cuts. Both materials are sound; they were simply placed in unsuitable conditions.

Hydraulic oils are also changing. Low‑viscosity oils, synthetic oils, and additive‑rich formulations place higher demands on material compatibility. FKM generally offers stronger chemical stability, while certain PU formulations may show volume changes in specific oils. This means material selection must consider not only the material category but also the exact oil type.

A practical engineering approach is to assign materials by location: FKM for high‑temperature static areas, PU for high‑pressure dynamic areas. The goal is not to choose the “superior” material but to address the dominant risk in the system.

If temperature is the main challenge, FKM is the better choice. If pressure and wear are the primary concerns, PU should come first. Matching the material to the working conditions matters far more than comparing the materials themselves.